Cushion and shrub ecosystem engineers contribute differently to diversity and functions in alpine ecosystems

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Abstract

Abstract Questions As ecosystem engineers, alpine cushion and shrub species have been widely documented, but their roles as co‐existing nurse plants have not been explicitly compared. In this study, our questions were: (1) what are the differences in engineering effects between cushions and shrubs in alpine ecosystems in southwestern China? (2) what are the differences in their influences on the attributes of alpine plant communities? and (3) can we detect the effects of changes in environmental stress with elevation on the engineering effects of cushions and shrubs? Study Site Three plant communities along an elevational gradient on Baima Snow Mountain, SW China. Methods Field measurements were carried out to assess effects of cushion ( Arenaria polytrichoides ) and shrub ( Rhododendron rupicola ) species on soil temperature, soil nutrient and water availabilities, the species richness of co‐existing vascular species and their abundance, above‐ground production and the percentages of grazed individuals. Results Shrub and cushion species modify micro‐environmental conditions, especially at higher elevations, but in different ways. Grazing pressures are highest for plants in the surroundings, followed by within cushions, and finally under shrubs. This indicates that both cushion and shrub species can protect beneficiary species from grazing by livestock. The presence of cushion plants increased species richness, abundance and above‐ground production in the higher community but not in the middle and lower communities. However, the presence of shrubs consistently increased those measurements across the elevational gradient. In addition, some species exclusively and some others preferentially occur within cushions or beneath shrubs. Conclusion The results clearly suggest that alpine cushion and shrub species in the Hengduan Mountains both act as ecosystem engineers and contribute positively to community attributes. However, their engineering effects vary with environmental stresses. Cushions could facilitate other species under severe abiotic stress, but showed much weaker or even competitive effects in benign conditions; shrubs’ facilitation may remain consistently strong under different stresses.

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